The present invention relates to a method for manufacturing a diffraction grating and a method for manufacturing a distributed feedback type semiconductor laser.
Adopted as a method for manufacturing a diffraction grating on a semiconductor substrate is forming a pattern directly on a resist via a direct patterning by means of the interference exposure or exposure via electric charge particles such as electrons and ions or forming a pattern on a resist by transferring the pattern formed on a mask. Among other things, the direct patterning method via the electron beam exposure enables forming of the precise, flexible and minute cycle-controlled diffraction grating which results in being excellent in manufacturing a distributed feedback semiconductor laser compared to a method for exposure via the interference exposure or a mask, but said method is inferior to the conventional method in terms of the throughput. To eliminate the disadvantage as described above, adopted is a method wherein when forming a diffraction grating by forming exposed patterns on a semiconductor substrate via an electron beam exposure, said diffraction grating is not formed on an entire surface of said substrate but the electron beam patterning line 1 is locally formed thereon by scanning and exposing electron beams to a region 20 to be the active region and the adjacent thereto only as shown in FIG. 1. This improves the throughput by locally forming the diffraction grating on the desired position of the substrate 4. The exposed patterns in this process are shown in FIG. 2.
However, if a diffraction grating is formed by the method shown in FIG. 1 and FIG. 2, the average height of a n-InP-substrate 4 in a diffraction grating forming region 5 is different from that of the non-diffraction grating 5a. This causes a stair difference on an epitaxial growth layer at the interface portion 5b between the diffraction grating forming region 5 and the non-diffraction grating forming region 5a if semiconductor layers, e.g. a n-InGaAsP guide layer 8, a n-InP spacer layer 9, a n-InGaAsP SCH layer 10, a strained multiquantum well (hereinafter referred to as xe2x80x9cstrained MQWxe2x80x9d) active layer 11, an InGaAsP SCH layer 12 and p-InP clad layer 13 are formed in their order on the n-InP-substrate 4 locally forming the diffraction grating in the (100) plane orientation. This stair difference portion causes the crystal growth plane direction to be a high dimensional plane displaced from the (100) plane. The crystal growth rate is thus accelerated and the material gas consumption is increased at the interface portion 5b between he diffraction grating forming region 5 and the non-diffraction grating forming region 5a. This provides an advantage to have the material gas density diluted at the adjacent interface portion and causes the crystal growth rate to be decreased. This also causes the composition of the diffraction grating to be fluctuated and the crystallinity of the growth layer is deteriorated due to the distorted stress resulted from the unaligned diffraction grating, the photo-luminescence half-width thus being increased as shown in FIG. 4. This crystallinity deterioration is resulted from that it is greatly deteriorated at the interface portion 5b and worsened at the center portion of the diffraction grating forming region 5 compared to that of the planar portion.
A great material gas density fluctuation at the interface portion 5b has an effect on the center portion of the diffraction grating 5 which results in having the crystal quality deteriorated compared to that of the planar growth portion. Thus, this deteriorates the quality of the active layer portion of a semiconductor laser which leads to not only the worsening of semiconductor optical output characteristics but also the increase of the threshold current as well as the efficiency deterioration. Furthermore, the crystallinity deterioration at the interface portion has an effect on the block structure in the case of DC-PBH-LD which results in increasing the threshold current and lowering the efficiency.
Also, the material gas density fluctuation not only increases the photo-luminescence half-width due to the crystallinity deterioration of the growth layer but also fluctuates the growth layer composition as well as the photo-luminescence wavelength. This raises a problem where it is difficult to control the band gap energy of the growth layer in addition to the strained MQW layer.
If the diffraction grating forming region 5 is adequately spaced to avoid the problem as described above, the throughput during the electron beam exposure is lowered and the manufacturing cost is greatly increased.
It is an object of the present invention to provide a method for manufacturing a diffraction grating and a method for manufacturing a distributed feedback type semiconductor laser, in which a high quality crystal growth layer can be formed on a diffraction grating which is locally formed by an electron beam exposure method.
Another object of the present invention is to form a strained MQW active layer wherein the band gap wavelength is not fluctuated on a planar portion and to manufacture a distributed feedback semiconductor laser with the excellent low threshold current characteristics.
A method for manufacturing a diffraction grating according to the present invention is to locally form the diffraction grating on a predetermined position of a semiconductor substrate including a process to coat a resist on an entire surface of the semiconductor substrate; a process to make electron-beam lithography of a diffraction grating pattern comprising straight line patterns to be reciprocally disposed in parallel with the predetermined position of the resist and make a local electron-beam exposure of said resist; a process to develop said resist; and a process to etch the semiconductor substrate using the developed resist as an etching mask wherein the resist used for the semiconductor substrate etching is all removed therefrom except from the diffraction grating forming region.
It is preferable that the resist left on the diffraction grating forming region only by the developing process be formed so that the resist-coated area may be reduced as said resist is away from the diffraction grating forming region. More particularly, a contour of the resist side end portion left on the diffraction grating forming region only may preferably be in the form of concave and convex, zigzag or sine function. Accordingly, if an etching mask pattern so designed as to have the resist-coated area gradually and uniformly changed is used, the average height of the semiconductor substrate resulted from forming a diffraction grating via a wet etching method is not discontinuously changed but gradually changed in the diffraction grating forming region and non-diffraction grating forming region. This is because the wet etchant consumption change is not steep at the interface therebetween. As described above, the gradual change of the average height of the semiconductor substrate enables the semiconductor layer in addition to the strained MQW layer formed thereon to become the same high quality crystal layer as that formed on a planar semiconductor substrate.
To form the diffraction grating as described above, a diffraction grating pattern is exposed by an electron beam exposure method on the adjacent region to be an active layer only during the process of forming a diffraction grating. Thereafter, the region which is not exposed by the electron beam is preferably exposed by a method using deep ultraviolet light in the wavelength range of 200-300 nm (hereinafter xe2x80x9cDeep UVxe2x80x9d or xe2x80x9cdeep ultravioletxe2x80x9d) and developed so that the resist may be left on the adjacent active region only and the resist-coated area can be gradually reduced from the resist-coated region to the non-resist coated region. Shown in FIG. 2 is the exposure pattern generated during the process of forming a diffraction grating of the present invention, for example, an exposure pattern is so structured that a diffraction grating pattern is formed on a region to be the active region and the adjacent thereto by a WAVE (Weighted-dose Allocation for Variable-pitch EB-corrugation) method and the region other than the diffraction grating forming region is then overlapped with the electron beam exposure pattern and exposed by the Deep UV light, the resist being removed from the region other than the active layer region and the adjacent thereto. Specifically, described below is a method for manufacturing a diffraction grating to achieve the objective as described above.
The method comprises a process to coat on an entire surface of a semiconductor substrate a positive resist to be exposed to both electron-beam and Deep UV light; a process to make electron-beam lithography of a diffraction grating pattern comprising straight line patterns to be reciprocally in parallel with the predetermined position of said resist and make a local electron-beam exposure of said resist; a process to expose with Deep UV light the region other than the electron-beam exposing region so that the interface between the Deep UV exposing region and non-Deep UV exposing region may be in the form of convex and concave; a process to develop said resist so that said resist may be left on a diffraction grating forming region only and removed from any region other than the diffraction grating forming region; and a process to etch said semiconductor substrate using said developed resist as an etching mask wherein the interface between the Deep UV exposing region and non-Deep UV exposing region may be in the form of a zigzag or sine function.
In addition to the foregoing method, described below is a method for manufacturing a diffraction grating comprising a process to coat on an entire surface of a semiconductor substrate a positive resist to be exposed to both electron-beam and Deep UV light; a process to make a local electron-beam exposure of said resist to form on said resist a pattern by reciprocally disposing in parallel on the predetermined position of said resist a number of the predetermined length of the first straight patterning lines and the second straight patterning lines each of which is shorter than each of said first straight patterning lines and which are patterned between said first straight patterning lines and at both ends thereof at several cycle intervals per said first patterning lines; a process to expose with Deep UV light the region other than the electron-beam exposing region so that the interface between the Deep UV exposing region and non-Deep UV exposing region may be in the form of a straight line; a process to develop said resist so that said resist may be left on a diffraction grating forming region only and removed from any region other than the diffraction grating forming region; and a process to etch said semiconductor substrate using said developed resist as an etching mask wherein a diffraction grating is locally formed on the predetermined position of said semiconductor substrate.
Unlike the method for manufacturing a diffraction grating as described above, the following is a method for manufacturing same using a negative resist comprising a process to coat on an entire surface of a semiconductor substrate a negative resist to be exposed to the electron-beam; a process to make a local electron-beam exposure of said resist to form on said resist a pattern by reciprocally disposing in parallel on the predetermined position of said resist the predetermined length of the first straight patterning lines and the second straight patterning lines each of which is shorter than each of the first straight patterning lines after the center of both first straight patterning lines and second straight patterning lines is aligned; a process to develop said resist so that said resist may be left on a diffraction grating forming region only and removed from any region other than the diffraction grating forming region; and a process to etch said semiconductor substrate using said developed resist as an etching mask and to locally form a diffraction grating on the predetermined position of said semiconductor substrate.
In addition to the foregoing, described below is a method for manufacturing a diffraction grating using a negative resist comprising a process to coat on an entire surface of a semiconductor substrate a negative resist to be exposed to the electron-beam; a process to make a local electron-beam exposure of said resist to form on said resist a pattern by reciprocally disposing in parallel on the predetermined position of said resist a number of the predetermined length of the first straight patterning lines and by disposing one or a plurality of the second straight patterning lines at several cycle intervals per said first straight patterning lines after aligning the center of both first straight patterning lines and second straight patterning lines each of which is shorter than each of the first straight patterning lines; a process to develop said resist so that said resist may be left only on a diffraction grating forming region and removed from any region other than the diffraction grating forming region; and a process to etch said semiconductor substrate using said developed resist as an etching mask and to locally form a diffraction grating on the predetermined position of said semiconductor substrate.
In this process, a chemically-amplified negative resist with the excellent sensitivity and resolution is suitable for forming a fine pattern.
The following is the description with respect to an additional method for manufacturing a diffraction grating using a positive resist comprising a process to coat on an entire surface of a semiconductor substrate a positive resist to be exposed to both electron-beam and Deep UV light; a process to make a local electron-beam exposure of said resist to form on the predetermined position of said resist a pattern comprising a number of the predetermined length of the straight patterning lines which are reciprocally disposed in parallel and the adjacent both ends of which are double-exposed; a process to expose with Deep UV light the region other than the electron-beam exposing region so that the interface between the Deep UV exposing region and non-Deep UV exposing region may be in the form of a straight line; a process to develop said resist so that said resist may be left on a diffraction grating forming region only and removed from any region other than the diffraction grating forming region; and a,process to etch said semiconductor substrate using said developed resist as an etching mask and to locally form a diffraction grating on the predetermined position of said semiconductor substrate.
In addition to the foregoing method, described below is a method for manufacturing a diffraction grating using a positive resist comprising a process to coat on an entire surface of a semiconductor substrate a positive resist to be exposed to both electron-beam and Deep UV light; a process to make a local electron-beam exposure of said resist to form on the predetermined position of said resist a pattern comprising a number of the predetermined length of the straight patterning lines which are reciprocally disposed in parallel wherein the adjacent both ends of one or a plurality of said straight patterning lines are double-exposed every other lines; a process to expose with Deep UV light the region other than the electron-beam exposing region so that the interface between the Deep UV exposing region and non-Deep UV exposing region may be straight line; a process to develop said resist so that said resist may be left on a diffraction grating forming region only and removed from any region other than the diffraction grating forming region; and a process to etch said semiconductor substrate using said developed resist as an etching mask wherein a diffraction grating is locally formed on the predetermined position of said semiconductor substrate.
Furthermore, the following is another method for manufacturing a diffraction grating using a positive resist comprising a process to coat on an entire surface of a semiconductor substrate a positive resist to be exposed to both electron-beam and Deep UV light; a process to make a local electron-beam exposure of said resist to form on the predetermined position of said resist a pattern comprising a number of the predetermined length of the straight patterning lines which are reciprocally disposed in parallel wherein the adjacent both ends of one or a plurality of said straight patterning lines are double-exposed at several cycle intervals per said straight patterning lines; a process to expose with Deep UV light the region other than the electron-beam exposing region so that the interface between the Deep UV exposing region and non-Deep UV exposing region may be straight line; a process to develop said resist so that said resist may be left on a diffraction grating forming region only and removed from any region other than the diffraction grating forming region; and a process to etch said semiconductor substrate using said developed resist as an etching mask wherein a diffraction grating is locally formed on the predetermined position of said semiconductor substrate.
A method for manufacturing a semiconductor laser of the present invention comprises locally forming a diffraction grating on the predetermined region of a semiconductor substrate and laminating and growing a semiconductor multilayer structure including an active layer on said semiconductor substrate wherein each of said methods of manufacturing a diffraction grating is adopted in the process of manufacturing said diffraction grating.
The description above is directed to the method for manufacturing a semiconductor laser of the present invention. According to a method for manufacturing a semiconductor laser of the present invention, it is possible to provide the same optical output characteristics and reliability as those of a DFB-LD with a semiconductor layer on a semiconductor substrate on the entire surface of which a diffraction grating is formed. Furthermore, each form of the diffraction grating formed by the interference exposure is randomly fluctuated and thus, there causes waveguide light loss in the DFB-LD with a diffraction grating formed by the interference exposure. However, an electron beam exposure method according to the present invention eliminates said problem and provides a high yield and low cost DFB-LD.
During the process of forming a diffraction grating for a DFB-LD of the present invention, a stripe resist is locally formed on a diffraction grating forming region of a substrate so that an area covered by the resist may gradually be reduced from the resist existing region to the non-existing region and thus, the area covered by the resist may gradually and uniformly be changed. Accordingly, the average height of the semiconductor substrate resulted from forming a diffraction grating via a wet etching method is not discontinuously changed but gradually changed in a diffraction grating forming region and non-diffraction grating. This results in providing a semiconductor layer formed on the diffraction grating which has same high quality as that of a semiconductor layer formed on a planar substrate. It is, therefore, possible to provide the excellent optical output characteristics and high reliability DFB-LD. In addition, there causes waveguide light loss in the DFB-LD according to a method for forming a diffraction grating via the interference exposure since the form of the diffraction grating is randomly fluctuated. However, an electron beam exposure method according to the present invention eliminates said problem and provides a high yield and low cost DFB-LD.